22 million Yahoo! Japan's user IDs may have been stolen during an unauthorised attempt to access the administrative system of its Japan portal.

“We don’t know if the file of 22 million user IDs was leaked or not, but we can't deny the possibility given the volume of traffic between our server and external terminals", Yahoo said.

The information did not include passwords and the kind of data necessary to verify a user’s identity or reset passwords, it said, adding that the company had updated its security measures to prevent a repeat of the incident.

In 2011, Sony said information such as usernames, passwords and birth dates of more than 100 million people may have been compromised after hackers struck the PlayStation Network and Sony Online Entertainment services.

Japan acknowledges that its preventative measures against cyberattacks remain underdeveloped, with the national police agency having announced this month it would launch a team to analyze and combat cyberattacks.

Japan Aerospace Exploration Agency said last month that information related to the International Space Station may have been leaked during an unauthorized attempt to access its system.